The outer membrane of Gram negative bacteria functions as a diffusion barrier, preventing periplasmic proteins from being diluted into the environment, and various deleterious environmental agents such as bile salts from gaining access to the periplasm. The necessary admission of small water-soluble nutrients and ions through this barrier ils mediated by passive diffusion pores composed of a few species of proteins called "porins." In E coli K-12 strains the predominant porins are OmpC and OmpF which may constitute more than 50% of the total protein mass of the outer membrane. However, when such a strain is lysogenized with PA-2, or some 1c+ containing derivative phage, the OmpF and OmpC proteins may be almost entirely replaced by a genetically distinct porin, protein-2 (also called the Lc protein). The structural gene for this porin has been shown to reside in the PA-2 genome and this replacement to be temperature dependent and influenced by 1c gene dosage. An increase in either of these parameters is accompanied by an increase in the level of protein-2, up to an apparent saturation level, and a concomittant decrease in the amount of OmpC and OmpF. This inverse relationship between the expression of protein-2 and that of OmpC and OmpF apparently allows the cell to maintain a relatively constant amount of porin protein in its outer membrane, the importance of which is not yet understood. The means by which the protein-2 (or some related gene) exerts its effect on the expression of OmpC and OmpF is not known, although experiments have indicated that this control is exerted at a post-translational level. It is the purpose of the proposed research to examine this novel form of regulation and to ascertain how the cell and/or phage regulates the coordinate expression of these porins. To reach this end we propose to isolate and characterize mutants defective in the expression of protein-2. This will entail mutant screening, genetic analysis, biochemical analysis, physiological characterization with promoter- and protein-fusion strains, and recombinant DNA technology.